Vessel for Rare Gas Filling, and Method for Polarization of Rare Gas Atomic Nucleus Using Said Vessel

ABSTRACT

A vessel for rare-gas filling is provided, where almost complete light circular polarization is realized, which has a light incident window of a single crystal material whose thickness and crystal axis orientation are optimized, and a polarization method of rare gas nuclei in the vessel is also provided. In addition, embodiments of the vessel is provided, which is impervious to alkali metal, and sustains high pressure, and shows no permeability for  3 He gas, and has negligibly small neutron absorption so as to be suitable for application to basic science, for example, neutron scattering, and the polarization method in the vessel is also provided. 
     The vessel for rare-gas filling of the present invention comprises a vessel body  3 , and a pipe  4 , which is connected to the vessel body  3  and introduces a rare-gas containing gas and an alkali metal into the vessel body  3 . A light incident window  2  made of a single-crystal material, which has proper thickness and proper crystal axis orientation, is attached to the vessel body  3 . The vessel for rare-gas filling is preferably made of sapphire or the like.

TECHNICAL FIELD

The present invention relates to a vessel for rare-gas filling, and a polarization method of the rare-gas nucleus using the vessel.

BACKGROUND

Magnetic resonance imaging (MRI) apparatuses have been used to examine the inside structure of an object without injury and damage. The MRI apparatuses are working and then playing an active role in medical image diagnoses along with X-ray CT at ordinary hospitals in local communications and so on. MRI uses nuclear magnetic resonance (NMR), where the interaction between nuclear spin and magnetic field is used. The resonance energy corresponds to a frequency region of several tens MHz (frequency region of FM radio transmission) in frequency. Therefore, electromagnetic energy of radiation on a test body in MRI is substantially lower than visible light and X-ray, and then MRI is less harmful. However, the low interaction energy in NMR/MRI means it has low detection sensitivity. Although the MRI diagnostic has been widely used, the resolution of MRI is lower than that of X-ray CT. MRI uses a hydrogen nuclear (proton, ¹H) spin, which has a larger interaction energy than other atomic nuclear spins, therefore, it visualizes the density distribution of hydrogen atoms of water or lipids in a living tissue. Therefore, the application to an organ such as lung, which has a low hydrogen density, is very difficult. Intense research works have been devoted to resolve this problem. High magnetic field and high efficiency pick-up coils have been applied to increase NMR detection sensitivity. However, these improvements are seamed to be in saturation. Innovation in basic technology, which is related with the principle of NMR, is required. The use of highly polarized rare gas nuclear spin will meet the requirement.

If the rare gas nuclear polarization at a normal pressure is increased, then the sensitivity of MRI is drastically improved. The rare gas nuclear polarization has been studied for application to basic science. The method thereof is as follows. A vessel is filled with rare gas and alkali metal such as rubidium or the like, which is irradiated with circularly polarized light. If we adjust the wavelength of the light to the D1 resonance of an alkali metal atom, then the electron spin of the alkali metal atom is polarized upon D1 resonant absorption. The polarized alkali atom collides with a rare gas atom, and then the electron polarization is transferred to a nuclear polarization through the hyperfine interaction between the electron spin and the rare-gas nuclear spin. The nuclear polarization obtained from this method is very high compared with the nuclear polarization in the ordinary MRI. When we apply this high nuclear polarization to MRI, the sensitivity will increase by a factor of several tens of thousands. A 1000 times more intense NMR signal will be obtained compared with the same volume of water, and therefore the polarized rare gas can be applied to MRI.

In the application to MRI, a polarized rare gas is carried to a sample in two ways. In one way, rare gas is stored in a container in a static magnetic field together with alkali metal vapor, which is irradiated with laser light. After the nuclear polarization by means of the laser light irradiation, the rare gas is carried to the sample. In the other way, a mixed gas of rare gas and alkali metal vapor is passed through a laser light irradiation vessel in a static magnetic field and continuously carried to a sample during rare gas nuclear polarization. The vessel in this polarization method is denoted as a flow-type polarization vessel. The sensitivity of MRI depends on the interaction energy of the nuclear spin with an electromagnetic field as well as the product of the square of the nuclear polarization and the number of polarized nuclei, which is same as in the application to basic science. This product is a figure of merit for optimization, which depends on various parameters, for example, light intensity in the D1 resonance, circular polarization, the spin exchange rate between electron spin and nuclear spin, spin relaxation times, a rare gas pressure, and an additional gas pressure of nitrogen or ⁴He gas, and so on. Higher light intensity and higher circular polarization are more preferable. Higher spin exchange rate, which depends on the combination of a rare gas and an alkali metal, and higher alkali-metal atomic number density, is of course more preferable. The alkali metal atomic number density depends on temperature. The relaxation times of electron and nuclear spins depend on the alkali-metal atomic number density, gas pressures, vessel wall condition and so on. Longer relaxation times are more preferable. As a result, suitable properties as the polarization vessel are as follows:

(1) the light incident window of the vessel should not attenuate the light intensity for the D1 resonance;

(2) complete circular polarization in the vessel should be realized in order to obtain a high nuclear polarization;

(3) the amount of paramagnetic impurities should be small on the surface of the vessel in order to obtain a long relaxation time;

(4) the vessel should be impervious to hot alkali-metal vapor;

(5) the vessel should sustain high pressure, because higher gas pressure, for example, more than several atmosphere is more preferable;

(6) the vessel has no permeability for ³He gas in the application to ³He polarization.

In addition, when the vessel is applied to basic science, especially precision experiments, it is required to have a certain accuracy,

(7) the thickness and material of the vessel should be uniform; and for an neutron scattering experiment,

(8) the material of the vessel should be transparent for neutrons.

Patent Document 1 describes an apparatus for producing a polarized rare gas in a flow-type polarization vessel of a coaxial, multi-cylindrical configuration, which comprises outer and inner cylinders made of silica glass with a clearance of 0.5 mm. A mixed gas of rare gas and alkali-metal vapor, for example, rubidium vapor, flows through the clearance, and the vessel is irradiated with the excitation light. A magnetic field is applied perpendicularly to the surface, which is irradiated with the excitation light. However, in the apparatus of the Patent Document 1, laser light intensity as well as circular polarization may largely decrease because of scattering from the vessel. Amorphous silica is used for both the outer and inner cylinders, which form the flow type vessel, and then circularly polarized laser light enters the aforementioned clearance through the outer cylinder of curved silica. Laser light scattering arises from the curved silica. Losses of laser light intensity and circular polarization decrease the rare gas nuclear polarization. The vessel should be chemically strong to hot alkali metal vapor as mentioned above, the glass vessel, which is used for the above application is not so strong to the hot alkali metal vapor, and therefore, it's life time is short. If we use ³He gas, the ³He gas permeates through the glass. In addition, the vessel for the rare gas polarization, which is made by means of glass blowing, has a problem of inaccuracy in the application to precision experiments of basic science, because the thickness and quality of the material are not homogeneous.

Patent Document 1: JP-A-H 11-309126.

Patent Document 2 describes a flow-type polarization vessel, where two flat glass disks are attached to the both ends of a glass cylinder. Gas pressure is raised for Doppler broadening of the resonance width, because a diode laser is used. The natural width of the D1 resonance is much smaller than the band width of the diode laser. The light incident window is flat in order to suppress the laser light attenuation at entering the vessel. However, the light incident window of the flat glass can not sustain high pressure, if it has not a sufficient thickness. The polarization vessel of the Patent Document 2 uses a thick disk of more than 5 mm thickness for the light incident window. In the polarization of rare gas nucleus, large circular polarization of the incident light is important. As described in the aforementioned document, the window should have no birefringence in order to keep the circular polarization. However, strain upon glass welding is serious issue. Birefringence may arise from the strain, which decreases the circular polarization, and in addition, it's effect increases in proportional to the window thickness. Furthermore, hot alkali-metal vapor adheres to the glass surface and then corrodes it. The alkali-metal atoms, which are located on the glass surface, will not be polarized, therefore, the rare gas nuclear polarization significantly decreases. For the ³He polarization in the flow-type vessel, the spin exchange rate should be higher. Potassium, in addition to rubidium, may be used for this purpose (E. Babcock et al., Phys. Rev. Lett. 91, 123003 (2003)). In this case, the corrosion becomes more serious. The permeation of ³He into glass may also cause a problem. The vessel of thick glass will not be suitable for basic science. The glass material in the Patent Document 2 contains boron, which absorbs neutrons. Neutron scattering experiments become impossible, because neutrons can not pass through the thick glass.

Patent Document 2: JP-A-2003-502132

Patent Document 3 describes a structure of flow-type polarization vessel, where quartz or sapphire, which has an excellent light transmission, is used for all parts or some part of the vessel as a light incident window. However, the Patent Document 3 explains only quartz or sapphire as an example of excellent light transmission material for a light incident window. There is neither description nor suggestion of the kind of crystal, such as single or poly-crystal and the orientation of crystal axis.

Patent Document 3: JP-A-2003-245263

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide a vessel for rare-gas filling and rare gas nuclear polarization by using the vessel, which can realize almost complete circular polarization in the vessel, which is impervious to alkali metal and can sustain high pressure, which is not permeable to ³He. As the material of the vessel and the light incident window, some kinds of single-crystal materials are used. The vessel has a negligibly small neutron absorption cross section. The vessel as well as the polarization method by using the vessel is suitable for application to basic science, for example, neutron scattering.

The inventor has intensively studied the problems mentioned above, and then completed a vessel for rare-gas filling and a rare gas nuclear polarization by using the vessel.

Principal features of the present invention are as follows:

(1) A vessel for rare gas filling, which comprises a vessel body, and an introduction port for rare gas and alkali metal, which is attached to the vessel body, and which has a light incident window made of a single-crystal material whose thickness and crystal axis orientation are properly adjusted.

(2) A vessel for rare-gas filling according to the item (1), which has a light incident window of birefringence.

(3) A vessel for rare-gas filling according to the item (1) or (2), which has a light incident window of sapphire or quartz.

(4) A vessel for rare-gas filling according to the item (1), (2) or (3), which has a light incident window of a flat plate.

(5) A vessel for rare-gas filling according to any one of the items (1) to (4), which has a vessel body and a light incident window, whose single-crystal material and crystal axis orientation are the same.

(6) A vessel for rare-gas filling according to any one of the items (1) to (5), which has a light incident window of birefringent single crystal whose principal crystal axis, c axis is parallel to the light incident window and thickness has a proper value. If incident light is linearly polarized, the c axis is rotated by 45° from the polarization direction.

(7) A vessel for rare-gas filling according to any one of the items (1) to (5), which has a light incident window of birefringent single crystal whose c axis is perpendicular to the light incident window or whose c axis is parallel to the light incident window and thickness has a proper value, where incident light is circularly polarized.

(8) A vessel for rare-gas filling according to any one of the items (1) to (7), which has a cylindrical vessel body, where a light incident window of disk is attached to one end of the cylindrical vessel body.

(9) A vessel for rare-gas filling according to any one of the items (1) to (8), which has an introduction port. The port comprises a first pipe part, which is connected to the vessel body or the flat plate part and made of the same single-crystal material as the vessel body, and a second pipe part, which is connected to the first pipe part and formed by joining a plurality of glass materials whose thermal expansion rates vary stepwise from small to large or vice versa.

(10) A method of polarizing rare-gas nucleus, which uses any one of vessel of the items (1) to (9). Light enters the vessel from a light incident window for rare gas nuclear polarization in a magnetic field.

Advantage of the present invention is that an almost complete circular polarization can be realized in the vessel for incident light. A light incident window of a single-crystal material, which has proper thickness and proper crystal axis orientation, is attached to the vessel body.

According to the present invention, a higher polarization can be obtained than before. In the conventional glass vessel, the polarization of light may decrease at passing through the glass, and therefore it is difficult to realize a high polarization in the vessel.

According to the present invention, the whole vessel can be made of sapphire or quart crystal, which can sustain high pressure and can be impervious to hot alkali metal vapor. Since neutron absorption cross section is very small for sapphire and quartz, neutron loss is small and background production is also small at neutron scattering experiments. It is easy to make the thickness of the crystal to be homogeneous for neutrons, which is suitable for precision experiments. Furthermore, the permeation of ³He into sapphire or the like is negligible.

In addition, according to the present invention, we can make a vacuum vessel, which can sustain thermal stress induced by temperature gradient upon welding the vessel body and the light incident window together. The vessel body and the window can be made of the same single crystal material. The crystal axis orientations of the vessel body and the window can be the same. The introduction port of rare-gas and alkali metal comprises the first pipe part of the same single crystal material as the vessel body, which is attached to the vessel body or the flat plate part, and the second pipe part, which is connected to the first pipe part. The second pipe part comprises a plurality of glass materials whose thermal expansion rates vary stepwise from small to large or vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and (b) are perspective views, which show two embodiments of the vessel for rare-gas filling according to the present invention;

FIG. 2 is a schematic view of alkali metal atomic and rare gas nuclear polarization;

FIG. 3 is a schematic view, which shows a vessel body, which has a light incident window and a pipe for introducing rare gas and alkali-metal vapor; and

FIG. 4 shows neutron polarization as a function of neutron energy.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the vessel for rare-gas filling according to the present invention will be specifically described below.

FIGS. 1( a) and (b) are perspective views, which show two embodiments of the vessel for rare-gas filling according to the present invention.

The vessel 1 for rare-gas filling shown in FIG. 1( a) comprises a cylindrical vessel body 3, two light incident windows of disks 2, 10, which are attached to both ends of the vessel, and a pipe 4 as an introduction port of rare gas and alkali metal into the vessel body 3, which is attached to the vessel body 3 or disk part 10 (in the figure attached to the vessel body 3). FIG. 1( b) show a vessel, which can be used for a flow-type polarization mode. The vessel has a gas discharge port 4 b as well as an introduction port 4 a.

The vessel 1 for rare-gas filling of the present invention can be used for rare gas filling, where incident light polarization can be controlled. The vessel can be also applied to the flow type polarization vessel of rare gas. The vessel can be apply to xenon and ³He, but the application is not limited to these gases. The rare gas containing gas comprises rare gas and a quenching gas such as nitrogen. In addition to the rare gas containing gas, it is necessary to introduce an alkali metal vapor into the vessel 1, but there is no limitation of the kind of alkali metal. As the alkali metal, rubidium, calcium, potassium, and sodium are included.

The light incident window 2 of single crystal material features in the present invention, which has a proper thickness and a proper crystal axis (c-axis) orientation, so that almost complete light polarization can be obtained in the vessel upon light incidence.

In the previous vessel as described in the Patent Document 2, non birefringent material is commonly used for a light incident window in order to avoid the depolarization of light. The present invention features a light incident window of a birefringent single crystal material, which has a proper thickness and a proper crystal axis orientation, so that almost complete light polarization is obtained in the vessel upon light incidence, whereby almost complete circular polarization is surely obtained in the vessel.

Examples of such single crystal, especially birefringent single crystal material includes sapphire and quartz.

The vessel of the present invention has a birefringent light incident window. The light polarization in the vessel can be precisely controlled by means of a proper crystal axis orientation and a proper thickness for the light incident window. When light propagates through a single crystal, a light oscillation phase shifts differently between two components, one component oscillates in the perpendicular direction to the c-axis, a principal optics axis (ordinary light), and the other component oscillates in the parallel direction (extraordinary light). The phase shift difference θ is represented as

θ=2π(n _(o) −n _(e))l/λ.

Here, n_(o) is a refractive index of the ordinary light, n_(e) a refractive index of the extraordinary light, l a light propagation, and λ a light wavelength. When the incident light is linearly polarized, the c-axis of the incident window is set perpendicular to the incident light, and the direction of linear polarization of the incident light is rotated by 45 degrees from the c-axis. The window thickness, which is the light propagation length, is adjusted so that

θ=(n+½)π,

then the linearly polarized light is transformed to a circularly polarized light at entering the vessel. Here, n is an integer. When the incident light is circularly polarized, the length l is adjusted so that

θ=(n+1)π,

then the circularly polarized light will be restored at entering the vessel. If we set the c-axis of the incident window parallel to the incident light, then the refractive index becomes independent of the light polarization direction, which is represented as n_(o). The light polarization does not change and the circular polarization does not change after the propagation. Thus the light polarization does not change for the circularly polarized incident light. The linear polarization is transformed to the circular polarization for the linearly polarized incident light. As a result, almost complete circular polarization is obtained in the vessel.

When a linearly polarized light beam enters the window 2, as shown in FIG. 1, the thickness of the window 2 is adjusted so that θ=(n+½)π, and the crystal axis (c-axis) is set parallel to the light incident plane of the window, where the light incident angle is 90°. The direction of the linear polarization is rotated by 45 degrees from the crystal axis (c-axis), then the linear polarization is transformed to circular polarization at entering the vessel after propagation through the window 2. As a result, almost complete circular polarization is realized in the vessel.

When a circularly polarized light beam enters the window 2, as shown in FIG. 2, the orientation of the crystal axis (c-axis) is set perpendicular to the light incident plane 2 a of the window 2, which is in the light propagation direction in the window 2, or the crystal axis orientation is set parallel to the light incident plane 2 a of the window 2, where the thickness of the window 2 is adjusted so that θ=(n+l)π. In these cases, the circularly polarized light enters perpendicularly to the light incident plane 2 a of the window 2. The circular polarization does not change at entering the vessel without. As a result, almost complete circular polarization can be realized in the vessel. In FIG. 2, a circularly polarized light beam, which is obtained from a linearly polarized light beam by means of a ¼ wavelength plate 5, enters the window 2. The vessel 1 is placed in a heating furnace 6 (oven) for alkali metal vaporization. The configuration will not be limited to such a case.

It is preferable that the vessel body and the window are made of the same single crystal material and they have the same crystal orientation.

The reason why a single crystal of sapphire or quartz is used for the light incident window 2 and the vessel body 3 is that it is mechanically and chemically strong, and its surface can be made clean, and in addition, it will sustain a pressure of 1 to 10 atmosphere and a temperature of 150 to 250° C. during polarization. These materials have small neutron scattering cross-sections, therefore, are suitable for application to neuron scattering. The light incident window 2 and the vessel body 3 are preferably made of the same single crystal, and also the crystal axes of the vessel body 3 and the window 2 are preferably aligned in the same direction so that they can sustain the thermal shock associated with a temperature gradient, because the light incident window 2 and the vessel body 3 are joined by welding.

In order to sustain the thermal shock on the vessel associated with the temperature gradient, the pipe 4 for introducing a rare-gas and an alkali metal (vapor) preferably comprises the first pipe part 7, which is attached to the vessel body 3 or the disk part 10 and is made of the same single-crystal material as the vessel body 3, and the second pipe part 8, which is connected to the first pipe part 7 and is formed by joining a plurality of glass materials whose thermal expansion rates vary stepwise from small to large or vice versa. For example, when the light incident window 2 and the vessel body 3 are made of sapphire, the first pipe part 7 is sapphire. The second pipe part 8 may be fabricated by welding a plurality (5 to 8 kinds) of glass materials so that the difference of thermal expansion rate from the value of the first pipe part 7 (e.g. sapphire) varies stepwise from small to large. In this case, a tip part 9, which is located in the opposite side of the pipe part 4 with respect to vessel body 3, is made of a glass (for example, Pyrex (registered trademark)) so that it is connected to a vacuum system or a rare gas and alkali metal filling system, and also the vessel can be baked in vacuum. For filling the vessel with a rare gas and an alkali metal, it is preferable that the window 10, which is made of the same single crystal material as the window 2, is attached at the other end of the vessel body by welding etc.

After the filling, the tip part can be melted and closed. The thermal expansion rates of the plurality of glass materials have differences from the value of sapphire, which vary stepwise from small to large so that failure upon temperature change can be prevented. At welding, the sapphire parts, crystal axis orientations of the first pipe part 7 and the vessel body 3 are aligned with each other since the thermal expansion depends on the crystal axis.

As a glass material for the tip part 9, borosilicate glass (heat resistant glass from Corning Inc.) or any equivalent glass is suitable for connection to a vacuum vessel, but it is not particularly limited. The pipe 4, as shown in FIG. 3, comprises the first pipe part 7 made of sapphire or quartz, which has an outer diam of 5 to 10 mm and an inner diam of 3 to 8 mm, preferably an outer diam of 5.5 mm and an inner diam of 3.5 mm, and the second pipe part 8 made of joined glass parts. The thermal expansion rate of the second pipe part varies stepwise from the value of the first pipe part 7 (for example, sapphire) to the value of the tip part 9 (for example, Pyrex glass) so that failure at welding is prevented.

The sapphire material, which is used as the parts of the vessel for rare-gas filling of the present invention, namely the light incident window 2, the vessel body 3, and the first pipe part 7, can be made by means of Verneuil's method. The parts are made by machining, grinding, and then fabricated by thermal diffusion method following a common procedure. On the vessel body 3 or the disk part 10, where the pipe 4 is attached, a proper hole for the pipe 4 is made. At the fabrication, the crystal axis orientations of the single crystal materials (for example, sapphire), which are used for the vessel body 3, the light incident windows 2 and 10 and the first pipe part 7, are determined by X-ray diffraction method so that all the crystal axes are aligned with each other.

The thickness of the single-crystal material, which is used for the light incident window of flat plate, is preferably 1 to 5 mm so that the window sustains high pressure with reasonable cost, if sapphire is used. A sapphire flat plate for the light incident window sustains high pressure with smaller thickness than a Pyrex flat plate. The thickness of the Pyrex plate should be more than 5 mm.

The next thing is the rare gas nuclear polarization in the present invention. In the rare-gas nuclear polarization, rare gas nuclei in the vessel 1 of the present invention, where light enters the vessel 1 through the light incident plane 2 a of the light incident window 2, are polarized in a magnetic field (not shown).

The polarization of the present invention uses optical pumping, which is explained as follows. Firstly, an atomic electron in outer shell is polarized. Here, an alkali metal atom, for example, rubidium (Rb) atom is used. The valence electron of Rb is in a 5S1/2 state. In a magnetic field, a spin state splits into parallel (−½) and anti-parallel (+½) spin states to the magnetic field. These two electron spin states exist in equal weight at thermal equilibrium. This situation is the same for nuclear spin states. The electron is excited from the s-state to the p-state by D1 resonant light of 795 nm in wavelength. If the light has right circular polarization, namely helicity +1, the angular momentum of the electron spin state changes by 1 at the light absorption because of angular momentum conservation. As a result, the electron spin state is excited from the parallel to anti-parallel spin state, while the transition from the anti-parallel to parallel spin state is forbidden. Therefore, only the parallel state of the s-state is excited to the anti-parallel state of the p-state. The excited state goes back to the original state (ground state) through a collision with a nitrogen atom with equal weight to the parallel and anti-parallel spin states because of spin state mixing. As a result, the population of the parallel spins decreases in the s-state, while the population of the anti-parallel spins increases, and then electron spins are polarized.

The alkali metal atomic polarization is transferred to the rare gas nuclear polarization via hyperfine interaction upon the atomic collision between the polarized alkali metal atom and the rare gas atom.

What has been so far described only shows some embodiments of the present invention, and various modifications may be made on the claims.

EXAMPLE 1

The vessel 1 for rare gas filling of the present invention is a colorless transparent vessel of single crystal material such as colorless, transparent birefringent sapphire or quartz, which is filled with a rare gas of 1-10 atmosphere and an alkali metal of 1 mg-1 g. For example, the vessel comprises a 30-100 mm long cylindrical vessel body 3, two flat plates, e.g. two 1-5 mm thick and 30-100 mm diam disks, which are welded to both ends of the vessel body by a thermal diffusion method as light incident windows 2 and 10, and a pipe 4 for rare gas and alkali metal filling, which is also welded on the vessel body 3 or the one of the flat plates. The filling pipe 4 is fabricated by welding together the first pipe part 7 of sapphire and the second pipe part 8 of a plurality (5 to 8 kinds) of glass materials, which has a tip part 9 of Pyrex glass so that it can be connected to a vacuum apparatus or a rare gas and alkali metal filling apparatus. The tip part can be melted and closed after the filling. The thermal expansion rates of the plural glass materials, which have intermediate values between sapphire and Pyrex glass, vary stepwise in order to prevent failure upon temperature change. At welding the sapphire parts together, their crystal axes are aligned since thermal expansion depends on the crystal axis.

In the vessel of the present invention, alkali metal atoms and rare gas nuclei are polarized by means of polarized light. During the polarization, the vessel is kept at a temperature around 200° C. for alkali metal vaporization, where polarized light enters. The light polarization is manipulated at the window 2 so that almost complete circular polarization is realized in the vessel. The reason why sapphire is used as a material for the light incident window of flat plate is that the sapphire is impervious to hot alkali metal vapor, and sustains high pressure in a form of thin plate, and has birefringence, and it's thickness and quality can be homogeneous. By using the birefringence of sapphire, the polarization of light, which enters the vessel through the light incident window of flat plate, can be precisely manipulated.

When the incidence light has a linear polarization, the crystal axis and thickness of the light incident window are adjusted so that linearly polarized light is transformed to circularly polarized light. The vessel 1 for rare-gas filling is fabricated so that the incident plane 2 a becomes parallel to the c-axis and the thickness of the light incident window satisfies the condition, θ=(n+½)π. If the direction of the incident linear polarization is rotated by 45° from the c-axis, and in addition, the light incident angle is set perpendicular to the window, then linear polarization is transformed to circular polarization in the vessel. Thus almost complete circular polarization is realized in the vessel. This is important to obtain a high nuclear polarization, since the alkali metal atomic polarization and also the rare gas nuclear polarization cannot exceed the value of the light polarization.

When the incident light has a circular polarization, the circular polarization of the incident light is held after entering the vessel, if the incident plane 2 a is set perpendicular to the c-axis. When the incident plane 2 a is parallel to the c-axis, the circular polarization of the incident light is restored, if the thickness of the light incident window is adjusted so that θ=(n+1)π. As a result, almost complete circular polarization can be realized in the vessel.

EXAMPLE 2

FIG. 2 shows an example of the alkali metal and rare gas nuclear polarization. For the alkali metal atomic or rare gas nuclear polarization, linearly polarized light from a diode laser or the like is transformed to circularly polarized light by means of a ¼ wavelength plate, and then enters a single crystal vessel filled with a rare gas and an alkali metal. The vessel is placed in the heating furnace (oven) 6 for alkali metal vaporization where a homogenous magnetic field of about 10 G is applied. The temperature of the oven 6 is about 200° C. The thickness and the crystal axis orientation of the window 2 of singe crystal material are adjusted so that the circularly polarization is held in the vessel. For example, as shown in FIG. 2, the circular polarization is held in the vessel, if the c-axis is perpendicular to the light incident plane 2 a. When the c-axis is parallel to the light incident plane 2 a, the circular polarization is restored, if the thickness is adjusted so that θ=(n+1)π. On the other hand, the vessel can be directly irradiated with the linearly polarized light. The linear polarization transformed to a circularly polarization at entering the vessel, where the thickness and orientation of the incident window are adjusted so that θ=(n+½)π and the ¼ wavelength plate is removed.

Circularly polarized light is absorbed by an alkali metal atom in the vessel, and then the electron spin of the alkali metal atom is polarized. Rare gas nuclear spin is polarized via hyperfine interaction upon atomic collision with the polarized alkali metal atom.

EXAMPLE 3

Here, it is shown that a high circular polarization for incident laser light is obtained in the vessel of the present invention, and then a high rare gas nuclear polarization is obtained. Rare-gas nuclear polarization P, is measured by means of neutron transmission T. The neutron transmission of the rare gas, which include the effect of the vessel, is represented as

T=Aexp(−σNd)cos h(PσNd).

Here, A is a neutron transmission of other materials such as vessel material and the like, except the rare gas, which is obtained from the experiment. Symbol σ is a neutron cross section of rare-gas nucleus, N the nuclear number density, and d a thickness of the rare gas along the neutron beam. When the nuclear spin is unpolarized, P=0, transmission T₀ becomes

T ₀ =Aexp(−σNd).

Since σ is obtained from a nuclear data table, the product of N and d is obtained from the neutron transmission. As a result, the nuclear polarization P is obtained from the measurement of T and T₀ by using the following equation,

P=cos h ⁻¹(T/T ₀)/σNd.

On the other hand, neutrons are polarized after transmission through the polarized rare gas. The neutron polarization P_(n) is represented as:

P _(n)=(1(T ₀ /T)²)^(1/2)

In Example 3, rubidium was used as the alkali atom to polarize ³He, and the neutron polarization was obtained from T and T₀. The result is shown as a function of neutron energy in FIG. 4. The ³He nuclear polarization was obtained from the neutron polarization via the ratio of T₀ to T. Here, the ³He polarization is obtained at many points of neutron energies, therefore, it is obtained with high precision by means of least square fitting. In FIG. 4, the circles denote experimental data and the solid line represents a theoretical curve, which is obtained by the least square fitting. The result of the ³He polarization is (63±1) %.

According to the theory of optical pumping, alkali metal atomic polarization P_(a) has the same value as the incident light circular polarization, if laser light intensity is enough. After the laser irradiation, the rare gas nuclear polarization increases up to a saturation value P_(o). P_(o) is represented in terms of the alkali metal atomic polarization and spin exchange rate γ_(se) as

P _(o) =P _(a)γ_(se)/(γ_(se)+γ)

Symbol γ is represented as 1/τ, a reciprocal of a time constant, which is a nuclear spin relaxation time. If the value of γ is much smaller than γ_(se), then the value of P_(o) becomes almost the same as the alkali metal atomic polarization. The value of τ is explained by collisions with rare gas atoms and impurities, which exist in the vessel wall and so on. When the laser light is switched off, the rare gas nuclear polarization exponentially decays as

P=P _(o) exp(−t/τ).

In Example 3, the value of τ was obtained to be 24 hours from the measurement of P as a function of time. The spin exchange rate γ_(se) was obtained as 1/γ_(se)=5 hours from the experiment by Baranga et al. (Phys. Rev. Lett, 80, 2801 (1998)).

In Example 3, 95% linearly polarized diode laser light was transformed to circularly polarized light by means of a ¼ wavelength plate. The circularly polarized light entered a 3.014 mm thick light incident window of a polarization vessel. Here, the circular polarization in the vessel is expected to be 87%. (The value is 95% at a thickness of 3.008 mm. This value is at a room temperature. The birefringence is desirable to be measured at 200° C.) As a result, the value of P_(a) is should be 87%, if the laser light intensity has enough value. The value of γ_(se)/(γ_(se)+γ) should be 83% according to the experimental values of γ_(se) and γ. Therefore, the rare gas nuclear polarization is expected to be 72%. In the experiment, the temperature inside the vessel becomes higher than the oven because of laser beam heating, and then the alkali metal atomic polarization may decrease at higher rate than expected, and therefore, the nuclear polarization becomes smaller. Therefore, the expected value of the rare gas nuclear polarization is consistent with the experimental value, which is 63%. If the laser light intensity is increased so that the polarization rate becomes much higher than the depolarization rate for the alkali atom, the experimental value approaches to the expected value. In conclusion, the circular polarization of the laser light is well manipulated by the thickness of the incident light window. As a result, a high circular polarization is obtained in the vessel by adjusting the thickness of the light incident window, and then a high rare gas nuclear spin polarization is obtained.

INDUSTRIAL APPLICABILITY

The polarized rare gases are applied to a tomography, MRI of a human lung or brain. The rare gas nuclear polarization is a key parameter, which increases the sensitivity of MRI. The high polarization by using the present invention is expected to contribute to the progress of MRI. The polarization of ³He, which is one of rare gas nuclei, provides an ideal neutron spin polarizer and analyzer. Large polarization with small neutron attenuation by using the sapphire or quartz of the present invention will improve the performance of the neutron polarizer and analyzer, thus greatly contribute to the exploration of material structures including biological materials by means of polarized neutron scattering. 

1. A vessel for rare-gas filling, comprising: a vessel body, and an introduction part connected to the vessel body and introducing a rare-gas containing gas and an alkali metal into the vessel body, wherein the vessel body includes a light entrance window made of a single-crystal material of which the thickness and crystal axis orientation are adjusted to be a predetermined thickness and a predetermined orientation to promote a high circular polarization and a high rare-gas nuclear spin polarization in the vessel.
 2. A vessel for rare-gas filling according to claim 1, wherein the light entrance window has birefringence.
 3. A vessel for rare-gas filling according to claim 1 wherein the light entrance window is made of sapphire or quartz.
 4. A vessel for rare-gas filling according to claim 1, wherein the light entrance window is a flat plate.
 5. A vessel for rare-gas filling according to claim 1, wherein the vessel body is made of the same single-crystal material as the window, and a crystal axis orientation of the vessel body is in the same orientation as in the window.
 6. A vessel for rare-gas filling according to claim 1, wherein when linearly polarized light is used as an incident light on the window, a single crystal having birefringence is used as a material for the window, and a direction of c-axis as one of crystalline axes forming a principal optic axis is made parallel to a light incident plane of the window, and the thickness of the window is made to a predetermined thickness, and on that basis, the linearly polarized light is entered perpendicularly to the light incident plane so as to incline its orientation at 45 degrees with respect to the direction of c-axis.
 7. A vessel for rare-gas filling according to claim 1, wherein when circularly polarized light is used as an incident light on the window, a single crystal having birefringence is used as a material for the window, and a direction of c-axis is perpendicular to a light incident plane of the window and the thickness of the window is made to a predetermined thickness, and on that basis, the circularly polarized light is entered perpendicularly to the light incident plane of the window.
 8. A vessel for rare-gas filling according to claim 1, wherein the vessel body is cylindrical and the light incident window is disc-shaped and is joined to one end of the cylindrical vessel body.
 9. A vessel for rare-gas filling according to claim 1, wherein the introduction part comprises: a first pipe portion located on a side connected to the vessel body or its flat plate part and made of the same single-crystal material as in the vessel body, and a second pipe portion connected to the first pipe portion and formed by joining a plurality of glass materials having different coefficients of thermal expansion to the first pipe portion such that the coefficients of thermal expansion vary stepwise from small to large for each of the plurality of glass materials in a direction away from the first pipe portion as they are joined.
 10. A method for polarizing rare-gas nucleus, comprising: obtaining a vessel as claimed in claim 1, and introducing light into the vessel from a light incident plane of a light entrance window to polarize rare-gas nucleus in a magnetic field. 